1. Field of the Invention
The present invention relates to an antenna and a wireless device incorporating the antenna. More particularly, the present invention relates to an antenna for mobile wireless communications which is especially useful in wireless devices such as mobile phone terminals, and a wireless device incorporating such an antenna.
2. Description of the Background Art
In recent years, technologies related to mobile communications, e.g., mobile phones, have seen a rapid development. In a mobile phone terminal, the antenna is a particularly important component. The trend for downsizing mobile phone terminals has required antennas to be downsized and also to become internalized elements.
Hereinafter, a conventional example of an antenna for mobile wireless communications, which may be used for a mobile phone terminal, will be described.
FIG. 16 schematically illustrates the structure of a conventional antenna for mobile wireless communications. As shown in FIG. 16, the conventional antenna for mobile wireless communications includes a conductive base plate 101, a conductive plate 102 of a planar configuration, and two metal leads 103 and 104. A predetermined voltage is supplied from a supply point 105 to the conductive plate 102 via the metal lead 103. Moreover, the conductive plate 102 is coupled to the conductive base plate 101, which provides as a ground (GND) level, via the metal lead 104.
An antenna of the above-described structure, commonly referred to as a PIFA (Planar Inverted F Antenna), is employed usually as a low-profile and small antenna device in a mobile phone terminal. The PIFA is a xcex/4 resonator, which is equivalent to a xcex/2 micro-strip antenna being short-circuited in a middle portion thereof to have its volume halved.
FIGS. 17A and 17B show current paths which emerge when a voltage is applied from the supply point 105 of the conventional antenna for mobile wireless communications shown in FIG. 16.
FIG. 17A shows a current path in an opposite phase mode. As shown by the arrows therein, the current path in the opposite phase mode begins at the supply point 105, extends through the metal lead 103 and along the lower surface of the conductive plate 102, and further extends through the metal lead 104 so as to be short-circuited to the conductive base plate 101. In the opposite phase mode, a current flowing through the metal lead 103 and a current flowing through the metal lead 104 do not contribute to the resonance of antenna because they have opposite phases and therefore cancel each other.
FIG. 17B shows a current path in an in-phase mode. As shown by the arrows therein, the current path in the in-phase mode begins at the supply point 105, extends through the metal lead 103 and along the lower surface of the conductive plate 102 so as to turn around at the open end, and further extends along the upper surface of the conductive plate 102 and through the metal lead 104, so as to be short-circuited to the conductive base plate 101. In the in-phase mode, a current flowing through the metal lead 103 and a current flowing through the metal lead 104 have the same phase at a frequency at which the length of the current path equals a xc2xd wavelength. Therefore, the antenna resonates at this frequency (referred to as the xe2x80x9cresonance frequencyxe2x80x9d).
FIG. 18 illustrates a detailed structure of the conventional antenna for mobile wireless communications shown in FIG. 16. As shown in FIG. 18, the conductive base plate 101 has a rectangular shape with a width of 40 mm and a length of 125 mm. The conductive plate 102 has a rectangular shape with a width of 40 mm and a length of 30 mm. The metal leads 103 and 104 are each 7 mm long. The volume occupied by the antenna (hereinafter referred to as the xe2x80x9coccupied volumexe2x80x9d of the antenna), which is defined within a region enclosed by an orthogonal projection of the conductive plate 102 on the conductive base plate 101, is equal to a product of the area of the conductive plate 102 and the lengths of the metal leads 103 and 104, i.e., 8.4 cc (=3xe2x89xa74xe2x89xa70.7), in this example.
In FIG. 18, the metal lead 103 functioning as a supply pin and the metal lead 104 functioning as a short-circuiting pin are shown with an interval of d therebetween. If the interval d is 3 mm, then the antenna shown in FIG. 18 will have a central frequency of 1266 MHz in the case of a 50xcexa9 system. Since the bandwidth (i.e., frequency bandwidth which has a voltage-standing wave ratio (VSWR) equal to or less than 2) under these conditions is 93 MHz, a band ratio of this antenna is calculated to be 7.3% (≈93/1266).
In the above-described conventional antenna for mobile wireless communications (PIFA), the resonance frequency and the length of the antenna element are generally in inverse proportion. Therefore, there is a problem in that the resonance frequency is increased if the length of the antenna element (i.e., the conductive plate 102), and hence the occupied volume of the antenna, is reduced in order to downsize the overall antenna.
Accordingly, there has been proposed an antenna structure for mobile wireless communications as shown in FIG. 19, which can provide a lower resonance frequency for the same occupied volume of the antenna.
As shown in FIG. 19, the conventional antenna for mobile wireless communications includes a conductive base plate 111, a conductive plate 112 of a planar configuration, a conductive wall 116, and two metal leads 113 and 114. A voltage is applied to the conductive plate 112 from a supply point 115, via the metal lead 113. The conductive plate 112 is coupled to the conductive base plate 111 via the metal lead 114. The conductive wall 116 is electrically coupled to the conductive plate 112 at one end thereof. Thus, the conductive plate 112 and the conductive wall 116 would together appear as if the conductive plate 102 in FIG. 16 was bent downward near its open end. A predetermined interspace exists between the other end of the conductive wall 116 and the conductive base plate 111. In this antenna structure, it is essential for the conductive wall 116 to be located at the farthest end of the conductive plate 112 from the metal lead 114.
The use of the above-described conductive wall 116 makes it possible to obtain a downsized antenna for the following two reasons.
First, an increased current path length lowers the resonance frequency. Specifically, the resonance frequency is lowered by disposing the conductive wall 116 so as to increase the maximum value of the current path length in the opposite phase mode (FIG. 20). Note that lowering the resonance frequency for the same occupied volume of the antenna is equivalent to downsizing an antenna while maintaining a constant resonance frequency. This is one reason why a downsized antenna can be realized by employing the structure shown in FIG. 19.
Second, the resonance frequency can be lowered due to capacitive loading. The interspace between the conductive wall 116 and the conductive base plate 111, which functions as shunt capacitance, is a factor in the lowering of the resonance frequency because the most intensive electric field resides at the open end of the conductive wall 116.
FIG. 21 illustrates a specific implementation example of the conventional antenna for mobile wireless communications shown in FIG. 19. Note that in the structure of FIG. 21, the dimensions of the conductive base plate 111 and the occupied volume of the antenna are the same as those of the structure of FIG. 18. In other words, the conductive plate 112 has a rectangular shape with a width of 40 mm and a length of 30 mm. The conductive wall 116 has a rectangular shape with a width of 6 mm and a length of 30 mm. The metal leads 113 and 114 are 7 mm long each.
If the interval d is 4 mm, then the antenna shown in FIG. 21 will have a central frequency of 1209 MHz in the case of a 50xcexa9 system. Since the bandwidth under these conditions is 121 MHz, a band ratio of this antenna is calculated to be 10.0% (≈121/1209).
However, while the above-described conventional antenna structure for mobile wireless communications makes it possible to lower the resonance frequency by bending the antenna element (i.e., the conductive plate) near one end, there is a problem in that its frequency band becomes narrower as the resonance frequency is lowered. As for the reduction in the antenna resonance frequency which is realized by narrowing the interspace between the conductive wall and the conductive base plate, there is also a problem in that any variation in such a small interspace would affect the impedance characteristics more substantially than a larger interspace, so that the stability of the characteristics is undermined. Moreover, due to limited design flexibility, the capacitive coupling between the antenna element and the conductive base plate is inevitably increased in a low-profiled antenna, which makes impedance matching difficult.
Therefore, an object of the present invention is to provide an antenna which can reconcile a low antenna resonance frequency and broadband frequency characteristics, while attaining stable impedance characteristics and high design flexibility; and a wireless device incorporating the antenna.
The present invention has the following features to attain the object above.
According to the present invention, there is provided an antenna for use in a wireless device, the antenna comprising: a conductive base plate for providing a ground level; an antenna sub-element disposed on the conductive base plate; an electromagnetic field coupling adjustment element which is electrically coupled to the antenna sub-element, the electromagnetic field coupling adjustment element being disposed so as to have a predetermined interspace with respect to the conductive base plate; and a supply connection member for applying a predetermined voltage to the antenna sub-element.
Preferably, the antenna further comprises at least one short-circuiting connection member for short-circuiting the antenna sub-element to the conductive base plate. The electromagnetic field coupling adjustment element may be disposed so as to produce an electromagnetic field coupling effect in conjunction with the short-circuiting connection member, or a portion of the electromagnetic field coupling adjustment element may be disposed in a direction generally parallel to the conductive base plate to produce an electromagnetic field coupling effect in conjunction with the conductive base plate.
The electromagnetic field coupling adjustment element may be disposed so that a maximum path from the supply connection member to the short-circuiting connection member is equal to xc2xd of a wavelength for a desired resonance frequency, wherein the maximum path extends so as to turn around an open end of the electromagnetic field coupling adjustment element not coupled to the antenna sub-element.
Thus, according to the present invention, an antenna element is designed in a characteristic shape having an electromagnetic field coupling adjustment element, so as to utilize electromagnetic field coupling with the conductive base plate. By adjusting the electromagnetic field coupling between the antenna and the conductive base plate through the adjustment of the dimensions of the electromagnetic field coupling adjustment element as parameters, it is possible to obtain a slight difference between the resonance frequency of the antenna and the resonance frequency of the conductive base plate, thereby providing broadband frequency characteristics. Moreover, the ability to produce a lowered resonance frequency also enables antenna downsizing without compromising broadband impedance characteristics. Since an increased number of design parameters is introduced, impedance matching is facilitated.
Preferably, all or part of a space surrounded by the antenna sub-element, the electromagnetic field coupling adjustment element, and the conductive base plate is filled with a dielectric material. As a result, a higher level of capacitive coupling between the electromagnetic field coupling adjustment element and the conductive base plate can be expected due to the dielectric material used for filling. Thus, further antenna downsizing can be attained.
Preferably, the electromagnetic field coupling adjustment element is fixed to the conductive base plate via a support base composed of a dielectric material. As a result, a higher level of capacitive coupling between the electromagnetic field coupling adjustment element and the conductive base plate can be expected due to the support base composed of a dielectric material, while being able to stabilize the antenna element provided on the conductive base plate. This also makes it possible to accurately control the distance between the electromagnetic field coupling adjustment element and the conductive base plate, so that an improved mass-productivity can be expected.
Preferably, a slit is provided in at least one of the antenna sub-element or the electromagnetic field coupling adjustment element for elongating the path from the supply connection member to the short-circuiting connection member. By providing such a slit, the resonance frequency can be lowered, and further antenna downsizing can be expected. In this case, a substantial decrease in the resonance frequency can be obtained by providing slits in regions associated with intense current distributions. It will be appreciated that providing slits in the electromagnetic field coupling adjustment element also helps in controlling the capacitance created in conjunction with the conductive base plate.
Preferably, the electromagnetic field coupling adjustment element and the antenna sub-element are formed as one integral piece through bending. Thus, by forming the antenna sub-element and the electromagnetic field coupling adjustment element from one integral piece, the mechanical strength of the antenna and the mass productivity of the antenna products can be enhanced.
Furthermore, the antenna according to the present invention may be configured so that the antenna resonates with at least two frequencies. That is, the antenna may comprise a plurality of the short-circuiting connection members (or supply connection members) which are specific to different respective resonance frequency bands. One of the resonance frequency bands may be selectively supported by controlling conduction of the plurality of short-circuiting connection members (or supply connection members). Thus, an antenna structure for selectively supporting two different resonance frequency bands with a single antenna can be realized.
The short-circuiting connection member may be specific to a first resonance frequency band, and the antenna may further comprise a slot specific to a second resonance frequency band. The two resonance frequency bands may be simultaneously supported based on the action of the antenna sub-element and the slot. Thus, the entire antenna element (i.e., the antenna sub-element and the electromagnetic field coupling adjustment element) supports a first resonance frequency band, while the slotted portion supports a second resonance frequency band. Therefore, an antenna structure which simultaneously supports two resonance frequency bands with a single antenna can be realized.
Two implementations of the antenna may be disposed on a common conductive base plate, wherein predetermined voltages are applied to the two implementations of the antenna with a phase difference of about 180xc2x0. Based on this configuration, not only the aforementioned effects are obtained, but it is also possible to concentrate currents flowing on the conductive base plate in the neighborhood of the antenna element. As a result, the device characteristics can be prevented from deteriorating when a device incorporating the antenna is held in one""s hand. By arranging the electromagnetic field coupling adjustment element so that the resonance frequencies of the two antennas are slightly different, more broadband-oriented characteristics can be expected.